This is a very important lesson: Always read and understand the terms and conditions of whatever product or service you purchase. For the airlines, each provides their Contract of Carriage on their website, and also make it available for review at the airline's ticket offices.
What you have come to realize is indeed the case with most major airlines, though the time period is either two hours or four hours, depending on the airline. And please understand that that is the only option the passenger ever has when their itinerary is changed by the airline: Getting a refund and trying to find other travel arrangements. Switching to yet-another (presumably more convenient) flight is a courtesy the airline may extend to the passenger, but they're never obligated to do so.
Also, remember, while JetBlue generally doesn't change things around quite so much, they also don't let you book your flights early. I'm STILL waiting to book my flights on JetBlue, five months after I made my hotel reservations, and I believe I'll be waiting at least another month. Meanwhile, if I wanted to book Delta, I'd already have locked in a maximum airfare.
